The present invention relates to on board diagnostics for vehicles, and more particularly to an engine off natural vacuum leakage check for a vapor handling system of a vehicle with an internal combustion engine.
In a conventional vapor handling system for an engine, fuel vapor that escapes from a fuel tank is stored in a canister. If there is a leak in the fuel tank, the canister or any other component of the vapor handling system, some fuel vapor can escape into the atmosphere instead of being stored in the canister. Leaks in the vapor handling system contribute to vehicle emissions.
In one approach set forth in U.S. Pat. No. 5,263,462 to Reddy, a controller that is connected to temperature and pressure/vacuum sensors monitors the vapor handling system. While the vehicle is soaking (engine off), the temperature sensor monitors the temperature in the fuel tank. If the temperature increases by a preselected temperature increment, a temperature switch changes state. The pressure/vacuum sensor monitors the pressure of the fuel tank and the vent lines and triggers a pressure switch if a preselected pressure is exceeded during soak. The pressure switch is set at a preselected pressure value that is lower than a threshold pressure of a pressure control valve. The pressure switch allows vapor to vent from the fuel tank to the canister.
At engine start-up, the controller checks whether the fuel tank experienced an adequate heat build-up during the soak. In other words, the controller checks whether the temperature switch was set while the engine as off. If the preselected temperature increase was not achieved, the switch is not set and the diagnostic leak check is not performed. If the temperature switch is set, then the controller determines whether the pressure switch is set. If the pressure switch is set, there is no leak in the system since the vapor handling system was able to maintain a preselected pressure. If the pressure switch is not set, then the vapor handling system could not achieve the preselected pressure because the vapors leaked into the atmosphere. The diagnostic system indicates the presence of a leak if the temperature switch is set during a soak and the pressure switch is not set.
Another approach measures a temperature decrease in the fuel tank while the engine is soaking and measures the fuel tank vacuum. A timer tabulates and stores the elapsed time that the engine is running. If the elapsed time is greater than a preselected time, the fuel tank was sufficiently hot before the soak. The engine coolant temperature is monitored at engine start-up. If the engine temperature is less than a preselected temperature, the fuel tank is cool. If the elapsed time is greater than the preselected time and the engine temperature is less than the preselected temperature, the fuel tank temperature decreased so that a vacuum should have been created in the fuel tank.
A vacuum sensor monitors the vacuum of the fuel tank and vent lines and sets a switch (vacuum) if a preselected vacuum is attained during the soak. If the vacuum switch was not set while the fuel tank temperature decreased, the controller diagnoses a leak in the vapor handling system.
The foregoing approach relies on a temperature sensor to provide temperature information for an ideal gas law math correlation. In use, it has been determined that there is no reliable correlation between temperature and vacuum due to the mass transfer between the liquid and the vapor in a fuel tank. Because the correlation is not reliable, the conventional temperature/pressure model is not valid for leak diagnosis.
Other conventional leakage diagnosis systems include a vacuum pulldown method that uses engine manifold vacuum and leak down rates to diagnose a leak. The drawback of this method is a lack of sufficient resolution to detect small leaks. In the near future, the government will require the detection of leaks on the order of 0.020 inch in diameter in vehicle vapor handling systems. The vacuum pulldown method cannot detect leaks this small. In addition, the vacuum pulldown method requires stiff fuel tanks. The vacuum pulldown method also has poor separation between good and failed data sets, which increases faulty detection rates.
Another conventional leakage diagnosis system uses a normally closed canister vent and measures vacuum over a relatively long period of time while the engine is off. One drawback to this method is the cost of additional hardware and the long test times that are required. Another engine off natural vacuum method assumes a mathematical correlation between temperature and vacuum build. Drawbacks of this method are the cost of the temperature sensor, lack of adequate correlation (resulting in poor prediction and poor data separation), and the inability to run the leak test in hotter ambient temperatures that are common in southwest United States.
A diagnostic method and system according to the invention for detecting leaks in a vapor handling system of a vehicle includes a fuel tank and a pressure/vacuum sensor that senses pressure and vacuum in the fuel tank. A canister recovers vapor from the fuel tank. A canister vent solenoid selectively provides atmospheric air to the canister. A controller connected to the canister vent solenoid and the pressure/vacuum sensor executes a leakage detection test that is capable of detecting leaks in the vapor handling system that have a diameter on the order of 0.020 inch.
In other features of the invention, the leakage detection algorithm generates data sets having greater than 25 standard deviations between leakage and no-leakage data sets. The leakage detection test includes a volatility test phase. The volatility test phase classifies a volatility of the vapor in the fuel tank into low, medium and high volatility. The leakage diagnostic test is aborted if the volatility is high.
In still other features, the leakage diagnostic test includes a pressure phase that is performed after the volatility test phase. During the pressure phase, the controller closes the canister vent solenoid and measures a pressure change in the fuel tank. If the pressure is increasing and the pressure change exceeds a pressure target value, the controller initiates an analysis phase. If the pressure is not increasing, the controller checks for a vacuum and performs a vacuum phase if the vacuum is present. If the pressure is not increasing and a vacuum is not present, the controller initiates the vacuum phase if the pressure remains zero for a first predetermined period.
In still other features, during the analysis phase, the controller opens the canister vent solenoid, sums an absolute value of a pressure change and an absolute value of a vacuum change, and initiates a reporting phase. During the reporting phase, the controller inputs the sum to an exponentially-weighted moving average, compares the exponentially-weighted moving average to a threshold, and declares a leak if the exponentially-weighted moving average exceeds the threshold.
In yet other features of the invention, during the vacuum phase, the controller opens the canister vent solenoid for a second predetermined period so that the vacuum phase begins at atmospheric pressure. The controller sets a vacuum target value equal to a total target value minus the pressure change measured in the pressure phase. The controller closes the canister vent solenoid and measures a vacuum change. If the vacuum is increasing and the vacuum change exceeds the target value, the controller initiates the analysis phase. If the vacuum is decreasing after a period of increasing vacuum, the controller initiates the analysis phase. If pressure is built, the solenoid is opened for a time and then reclosed to attempt the vacuum phase. If the vacuum is zero for a second predetermined period, the controller initiates the analysis phase.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.